But the issue gained traction around Twitter, and many argued that it's not diversity, but "bad writing and lack of imagination" that's hurting the comic book giant.

Both Marvel and DC Comics have been pushing for more ethnic and gender diverse characters to gain leading roles in both the books and the big screen.

In 2011, Miles Morales became the first black Spider-Man in Marvel history.

It was quite a feat, considering most black superheroes like the Black Panther, Cloak or the X-Men Bishop took more secondary roles - there had never been a black hero as important as Spidey.

A female Thor followed in 2015, even outselling the manly God of Thunder.

Since then, Miss Marvel was made Muslim and Iceman joined the likes of Northstar, coming out as gay.

DC Comics made a similar effort, even writing an editorial titled "Diversity Matters", reminding readers that Catwoman was bisexual and Wonder Woman officiated a gay wedding.

In the movies - comic book related or not - studies have been published claiming the lack of diversity is costing the industry billions.

"The industry's homogeneous corps of decision makers made relatively few of these types of diverse projects, potentially leaving billions in revenue on the table," said Dr Darnell Hunt, author of a study called 2016 Hollywood Diversity Report.

Another study, from the University of South Carolina, found that 30% of Hollywood's speaking characters were women last year, and only around 2% were lesbian, gay or transgender.

Late last month, Disney's Beauty And The Beast - which featured a feminist hero and a gay character - broke box office records, turning fuss to benefit.

As for ethnic minorities, only 12.9% took on leading roles in 2014.

Hunt told Variety magazine that "most of the movie tickets for four of the top 10 films in 2014 were purchased by people of colour", who didn't see themselves represented.

As for the music industry, it is something of an untouchable - at least when it comes to diversity in talent.

But there is no denying that, while the Marvel Cinematic Universe keeps expanding, its main publishing branch is in crisis.

"There was just a big shift in the entire industry, and there were a lot of factors behind that," Mr Gabriel said.

"There was anger because of economic reasons. There was anger because of story reasons for all of us."

He argues that retailers want more of the old characters back because they sell more, and it's a fair point - but one that shouldn't stop a push for diversity.

The truth is, when film, music, the remaining elements of pop culture and culture in general, were only representing white men, comic books were already one step ahead.

In 1941, when women in the US were still struggling to get a job as a receptionist, Marvel released the eighth issue of its All Star Comic, where a warrior Amazon Princess is tasked with saving the Americans from the Nazis.

Northstar, the Uncanny X Men, took most of the 1980s to come out as gay, but did so in 1992, six years before what US journalist Frank Rich described as the "homophobic epidemic of '98".

Comic books have always been in the front line of diversity, giving different readers different heroes to choose from.

Imagine a world ruled by Captain America and Superman - wouldn't that be dull?